1. Field of Invention
The present invention relates to a steering system for a commercial vehicle; and, particularly, to a hybrid motor driven power steering system for a commercial vehicle, and a method of improving performance and fuel efficiency through control thereof. The steering system is capable of greatly increasing fuel efficiency as well as satisfying a steering capacity of about 50 KN to 190 KN required in small-sized, medium-sized, and full-sized commercial vehicles by constructing a ball nut type gear box together with a Motor Driven Power Steering (MDPS) unit and an Electro-Hydraulic Power Steering (EHPS) unit controlled by a main steering control device, and capable of significantly improving the driver's steering operability by constructing two torsion bars for the respective MDPS unit and EHPS unit.
2. Description of Related Art
A steering device generally needs a hydraulic pump because of using hydraulic pressure as a steering assist force. The hydraulic pump is driven by an engine, thereby causing loss of an engine driving force. This loss of the engine driving force results in deterioration of fuel efficiency. Accordingly, if the loss of the engine driving force is reduced through the steering device, the fuel efficiency may be improved corresponding to the reduced loss.
As an example of improvement in fuel efficiency, there is a hydraulic steering device, though having the hydraulic pump driven by the engine, capable of reducing the loss of the engine driving force by decreasing resistance in a hose or a flow rate of the pump during idling of the engine. The improvement in fuel efficiency using the hydraulic steering device, however, is not effective due to the hydraulic pump driven by the engine. To overcome this problem, the hydraulic pump should not be driven by the engine.
As a steering device from which the hydraulic pump is not driven by the engine, there may be a Motor Driven Power Steering (MDPS) device which uses an electric motor or an Electro-Hydraulic Power Steering (EHPS) device which uses an electric pump driven by a motor instead of an engine driving force. Therefore, the MDPS or EHPS device is used as a steering device of a hybrid passenger vehicle which absolutely needs improvement in fuel efficiency.
In addition to the passenger vehicle, the fuel efficiency improvement should be achieved also in a commercial vehicle. Thus, if the MDPS or EHPS device is used as the steering device for the commercial vehicle without the hydraulic pump using the engine driving force, the fuel efficiency of the commercial vehicle may be significantly increased.
The MDPS or EHPS device, however, has a limit in steering capacity. That is, the MDPS or EHPS device may be suitable for the passenger vehicle which needs the steering capacity of about 13 KN, but may not be suitable for the medium-sized or full-sized commercial vehicle which needs the steering capacity of about 50 KN to 190 KN. For this reason, in the case of solely applying the MDPS or EHPS device to the commercial device to increase the fuel efficiency, scarcity of the steering capacity must be solved first.
The MDPS and EHPS devices together with a rack and pinion type gear box may be used as the steering devices adapted for increasing the steering capacity of the commercial vehicle. But difficulty remains in using the devices for the medium-sized or full-sized commercial vehicle which needs the steering capacity of about 50 KN to 190 KN. Meanwhile, when the MDPS and EHPS devices are used as the steering devices, a phenomenon is generated in which twisting felt by a driver differs from real twisting of a steering column. Consequently, this phenomenon must be solved.
To this end, one torsion bar is applied to the steering devices and is installed to a region of the gear box to reflect a road surface reaction force transferred to the gear box, but the reflection may not be effective. The basic reason is because of the layout of the MDPS and EHPS devices. For example, in the layout of the steering device, each of the region of the steering column to which the MDPS device is mounted, the region of the steering column to which the EHPS device is mounted, and the region of the joint (universal joint) to connect the steering column have different stiffness, and difference in stiffness between the regions generates a torque fluctuation rate at the region of the joint (universal joint). Thus, the twisting felt by a driver may differ from the real twisting of the steering column.
The MDPS and EHPS devices may be controlled in a state of exactly reflecting the road surface reaction force due to difference in twisting as described above, thereby causing performance deterioration of the steering device. Therefore, in order to apply the MDPS and EHPS devices, which may help in improving fuel efficiency, to the commercial vehicle requiring high steering capacity of about 50 KN to 190 KN, it is necessary to solve the phenomenon in which the twisting felt by a driver differs from the real twisting of the steering column.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.